The present invention is directed to a hammer drill containing a cylinder forming a piston chamber in which a drive piston is reciprocated by a motor. and a percussion piston within the piston chamber is similarly reciprocated due to an air cushion located between the pistons.
In known hammer drills with a so-called pneumatic percussion mechanism including a drive piston, reciprocated within a piston chamber in the cylinder by a motor, and a percussion piston, reciprocated by an air cushion located between the pistons, a sufficient air cushion may not be available due to increased air leakage losses with resultant damage to the percussion mechanism and also to the drive elements of the percussion mechanism and to other parts of the device, if the pistons strike against one another. The leakage losses result from insufficient sealing between the piston and the cylinder forming the piston chamber, particularly due to wear of the piston sealing elements. Further, inadequate lubrication and the failure of the sealing elements, which may be heat sensitive, tends to cause leakage losses.
To avoid expensive damage beyond the percussion mechanism due to leakage losses, in a known hammer drill disclosed in German Offenlegungsschrift No. 2 729 596, a base part is provided in the drive piston which collapses when it impacts against the percussion piston. As a result the air flows out of the air cushion space between the two pistons so that the percussion piston can no longer be reciprocated.
A disadvantage of such an arrangement is that damage to the parts of the hammer drill may occur when the two pistons impact against one another, which impact is necessary to collapse the base part.